1. Field of the Invention
The present invention relates to a dielectric thin film with high performance used in various electronic devices, display devices, light modulator devices, and so on. The invention also relates to a thin-film electroluminescent (EL) device using such a dielectric thin film.
2. Related Arts
In recent years, LSI and display device fabrication techniques have evolved. At the same time, there is an increasing need for thin-film materials having high dielectric constants and good insulation. That is, these thin-film materials are used in high-dielectric-constant capacitors contributing to miniaturization of LSIs and in high-dielectric-constant dielectric films used for large-sized, highly reliable display devices. More specifically, a transparent dielectric film of a high dielectric constant is formed on a transparent substrate. A functional film is formed on the dielectric film. Thus, fabrication of a display device comprising the transparent glass plate is attempted such that characters are made to emerge from the transparent glass in operation. Also, fabrication of an optical modulator device for adjusting the intensity of light transmitted through the glass is attempted. In this way, application of thin-film materials of high dielectric constants has been earnestly studied. Among these various applications, in the field of electroluminescent (EL) devices, there is a need for thin-film materials having higher dielectric constants and good insulation.
Among EL devices, a fully solid-state thin-film EL device is a display device which has excellent durability, self-emitting property, and excellent visibility. This solid-state thin film EL device has been put into practical use as a flat panel display. Furthermore, a thin-film EL device uses a pair of electrodes each made of a transparent conductive film, whereby a transmissive type light-emitting device can be built. In this manner, thin-film EL devices are highly promising light-emitting devices and expected to find wide application.
A very high alternating electric field is applied to the thin-film EL device because of the principle of operation of the device. Therefore, the thin-film EL device has the disadvantage that the dielectric breakdown of the dielectric layer of high dielectric constant limits the life of the EL device. Accordingly, if a thin film having a high dielectric constant and good insulation can be formed in the thin-film EL device, then long life can be imparted to the device. Also, the device can be made to emit stably and efficiently. As a result, thin-film EL devices can be manufactured with improved yield. Also, the area of the emitting surface can be increased.
In the past, dielectric films of the above-described thin-film EL device have been made of silicon dioxide, alumina, silicon nitride, yttrium oxide, and other materials. Since dielectric films made of these materials have small relative dielectric constants, it is impossible to apply an effective voltage to the light-emitting layers. Hence, a higher driving voltage is required. Furthermore, it has been attempted to fabricate a dielectric layer for use in the thin-film EL device from tantalum oxide whose relative dielectric constant is 5 or 6 times as high as that of silicon oxide. However, if thin films of tantalum oxide and transparent conductive films as consisting of ITO (indium tin oxide) are stacked on top of each other, then the dielectric strength deteriorates drastically. Accordingly, methods for making a multilayer dielectric structure by placing a thin film of silicon dioxide, alumina, silicon nitride, yttrium oxide, or other material at the interface between a dielectric film of tantalum oxide and a transparent conductive film have been proposed (Japanese Patent Laid-Open Nos. 50-27488, 54-44885, 56-52438, and 58-216391). Nevertheless, these multilayer dielectric films have failed to yield great advantages. Furthermore, complicated manufacturing steps have been necessitated.
It has also been attempted to fabricate a thin film of improved dielectric strength by adding yttrium oxide, tungsten oxide, or other material to a film of tantalum oxide (Japanese Patent Laid-Open No. 4-366504). Indeed, the dielectric strength of the dielectric thin film can be improved, but the aforementioned problem remains to be solved, i.e., if these dielectric thin films and transparent conductive films made of ITO or the like are laminated on top of each other, then the dielectric strength drops drastically.